A toroidal drive, having a continuously variable ratio, which has at least two pulleys with toroidal facing surfaces between which friction rollers roll to transmit a torque from one pulleys to the other, are well known. One of the pulleys is connected with an input shaft and the other with an output shaft. To change the ratio, the axes of rotation of the friction rollers relative to the input axis are swivelled so that the friction rollers on the input pulleys move on a contact radius which is different from the contact radius on the output pulleys. The ratio to each other of the contact radii gives the reduction ratio.
DE 28 07 971 A shows a simple toroidal drive of that kind in which the friction rollers are supported in a holding device and swivelled via an axially movable drum.
EP 0 415 391 B1 has disclosed a toroidal drive in which, two friction rollers diametrically disposed relative to an input shaft and between an input pulleys and an output pulleys, are disposed which have semi-toroidal surfaces opposite each other. The friction rollers are supported on eccentric shafts of two roller carriers which are pivoted in a holding device connected with a transmission housing. By means of hydraulic setting devices, the roller carriers can be adjusted in direction of their swivel axis wherein they are simultaneously tilted. Since the friction rollers are situated diametrically to the input shaft, they are swivelled in opposite directions, that is, when one roller is swivelled clockwise, the other is swivelled counterclockwise.
The hydraulic setting device has a double-acting piston, the sides of which are loaded by a pressure difference, which is produced by a valve unit having a piston slide valve with four leading edges. A control unit adjusts the piston slide valve, via control springs, by means of a stepping motor and thus adjusts the reduction ratio desired. A mechanical return system, which contains a cam and lever mechanism, determines the tilting movement of the roller carrier, and acts likewise upon the piston slide valve and ensures that the adjusted reduction ratio be maintained.
The valve device has a strong, non-linear reinforcement of the pressure difference relative to the ratio divergence which can only be overcome with difficulty and particularly, in case of extremely high ratios, for example, by geared systems. Thus yields driving conditions hard to control when starting. In such transmissions the continuously variable reduction step, the so-called variator, is situated in a power branch of a power distribution transmission and can be regulated so that in an operating state the output shaft of a cumulative steps assumes the speed zero and thus stands a vehicle with such an input. During very low output speeds, very high torques appear which, even if desired for starting, cannot be braked by the service brakes when the vehicle is parked and thus result in creeping of the vehicle. In addition, the speed sensors required for regulating the speed are not capable of determining any low speeds which strive against zero.